The stimulation of oil or gas production by injecting a diluted acid solution into a nonproducing or damaged formation is known as acidizing. The diluted acid solution enlarges the flow channels by dissolving corrosion products, scale, mud and/or a portion of the formation. Then, the oil or gas passes through to the wellbore increasing the well's production.
It is well recognized that oil/water separation becomes significantly more challenging when production fluids from acid workovers are introduced into the system. Following are some of the causes and prior solutions to water treating problems during upsets induced by acid flowbacks.
The cause of the oil/water separation problem is generally attributed to the following factors:
1. poor dissolution of oil-external latex polymers in low pH water; PA1 2. increased polymer demand due to solids; and PA1 3. solids build-up at the interface in oil-treating vessels.
All these factors contribute to carryover of oil and/or unresolved emulsion into the water. Under normal conditions, high charge, high molecular weight oil external latex polymers are used for water clarification. However, these polymers have reduced solubility during acid flowbacks. These oil-external solutions must "invert", that is, the polymer must dissolve into the water, and the polymers chains become disentangled for maximum efficiency. Increased agitation and residence time can assist in this inversion process, but often during acid flowbacks a small amount of the latex polymer is left uninverted. As chemical dosages are increased to handle elevated levels of contaminants there is generally a maximum level, above which uninverted polymer can actually contribute to chemical sheening as you have observed. Thus, solubility of the polymer is a factor in selection of the optimal product.
Dissolved species, including various silicates, carbonates, and oxide/hydroxides of iron and calcium, can become destabilized as the pH of the solution rises, either as the solution becomes "spent" or is co-mingled with produced water buffered at a higher pH. Furthermore, solids, including sands and silt and scale particles can also be dispersed into the water during the treatment. The resultant increase in solids, often oil-coated, can create a significant increase in the polymer demand of the system, or create a need for a different coagulant and/or flocculant treatment to effectively clarify the water.
Furthermore, these solids contribute to the formation of tight emulsions during the acid workover. The same solids, oil-wetted, often accumulate at the interface in treating vessels, building pads or severely irregular interfaces. As these can be drawn into the water dump from these vessels, carryover of dispersed or emulsified oil into the water can occur.
Sheening can often occur, even though residual oil is within discharge specifications. The sheening may be linked to very fine solids, often oil coated, which are not removed in the water clarification stage. For example, soluble iron species can precipitate as they become exposed to the oxygen-rich ocean waters, serving as a seed to flocculate the remaining low-level oil. Sheening has also been attributed to uninverted polymer which can carry through the system, but continue to dissolve as it dilutes into the ocean water, again flocculating the very low levels of oil in the discharge water, or releasing the low level of hydrocarbon solvents contained in the polymer.
There are several components of the drilling program which are carefully designed to mitigate these problems. Acetic acid which is used to buffer the water at an intermediate acidity to keep many of these species in solution until they can pass through the system, to minimize extremes of pH. At very low pH, precipitation of asphaltenes is aggravated, while at high pH's, various scalants will more rapidly precipitate. Acetic acid, along with chelants, also help to keep various iron species in solution.
Nonionic surfactants are used to minimize the generation of solid-stabilized emulsions during the drilling and acidizing. Prevention of these emulsions, by addition of these non-emulsifiers, is one treatment to remove the residual oil from the water, but is not sufficient.
The low pH and solids discussed above contribute to increased polymer demand for the water clarification system. Under these conditions, highly charged oil-external latex polymers are no longer the most efficient treatment. The polymers of the invention are generally more efficient water clarifiers, often requiring substantially lower dosages to achieve required water clarity. The polymers of the invention have been demonstrated to function during such pH and solid upsets. These polymers are typically far more soluble in brine, especially at lower pH, and can be fed at higher levels to meet the increased polymer demand during these upsets.
The water-soluble dispersion polymers of the invention offer many solutions to these problems and represent a new method for recovering oil from water during or after an acid workover.